1. Field of the Invention
The present invention relates to an image correction method for correcting image defects due to ejection-amount nonuniformity, deviation in a landing position (kink), and nonejection, which are inherent characteristics in each recording head of an inkjet recording system, in which by ejecting ink, ink dots are formed on a recording medium so as to form an image thereon.
2. Description of the Related Art
As copying machines, information processing equipment such as word processors and computers, and communication equipment achieve increasing popularity, digital image-recording apparatus using inkjet recording heads have also gained widespread use and acceptance. Enhancements to image quality and color in information processing equipment have led to the need for corresponding enhancements to image quality and color in image forming apparatus.
Such a recording apparatus utilizes a recording head integrated with plural recording elements (also referred to as a multi-head) in which plural ink nozzles and ink paths are integrated in high density for miniaturizing and speeding up printing a pixel. Furthermore, for colorization, the apparatus generally has plural multi-heads corresponding to the respective colors of cyan, magenta, yellow, and black. Using this design, it is possible to output high quality images at both high speed and low cost. Another practical way to increase speed ever further is to use a one-pass high-speed method, in which the length of the multi-head is about the width of a recording medium.
In a transverse-feed printer for A-4 size paper, for example, the length of a multi-head is about 30 cm, and requires approximately 7000 nozzles to print 600 dots per inch (dpi). It is extremely difficult to manufacture a multi-head having such a large number of nozzles without defects in one or more of the nozzles. Accordingly, all the nozzles may not necessarily have the same performance. Furthermore, some nozzles may become nonejectors after being used. However, a recording head shading technique for correcting density nonuniformity due to ejection-amount nonuniformity and deviation in a landing position (kink), and a nonejecting-nozzle correction (nonejection complementary) technique for performing complementary processing for a nonejecting nozzle can enable a multi-head with defects to be used.
According to one recording head shading technique, the output density of every nozzle is measured and input-image data gets feedback from the measured result. For example, if the ejection amount of one nozzle is reduced for some reason so as to reduce the output density of a particular nozzle, the recording head shading technique adjusts the input image so that a gradation value in a portion corresponding to the affected nozzle is increased so as to have uniform image density in the output image.
As a nonejection complementary technique, if one nozzle is nonejecting, there are compensatory methods, such as substituting the ejection of nozzles on the both sides for the dot to be ejected by the nonejecting nozzle (adjacent complementing), or complementing data corresponding to the nonejecting nozzle with an ink dot of another color such as black (different-color complementing).
Although the aforementioned recording head shading and nonejection complementing methods are effective for improving recorded-image quality, these techniques are not without problems.
For example, if the amount of ink ejected from some nozzles in a recording head is decreased so as to reduce overall density, by increasing gray scale intensity in the affected portion, the recorded image will appear to have uniform image density (shading correction). However, if a nozzle with decreased ejection ability is printing in a region requiring full discharge capacity (duty factor of near 100%), no additional compensation above the nozzle""s maximum decreased capacity is possible. Therefore, correction of this region is difficult to perform.
Similarly, in the adjacent complementing method, in which a nonejecting nozzle is complemented with an adjacent nozzle, if a portion adjacent to the nonejecting nozzle has a recording duty factor of 100% or close thereto, because the density of the adjacent portion cannot be further increased, the nozzles adjacent to the nonejecting nozzle will be unable to compensate.
In order to contend with such a problem, the inventors of the present invention have proposed a method for correcting a nonejecting nozzle, in which a nonejecting nozzle is corrected by a different recording head so as to minimize differences in lightness or color difference using a color different from the nonejecting nozzle. As to the recording head shading method, no countermeasure has yet been proposed.
Another compensation method involves virtually increasing the resolution (recording density) of a recording head in a relative principal scanning direction (transferring direction in a case that a recording medium is transferred with a recording head fixed) is virtually increased so as to enable the gray scale in the entire gradation regions to be corrected by enabling the recording medium to be recorded thereon by 100% or more as in a conventional method. However, according to this method, the amount of the data fed to the recording head is increased, resulting in a decrease in the per page recording rate. Furthermore, since the number of recording dots per unit area is increased, the ejecting frequency needs to be further increased in order to maintain the recording rate. Since the printing operation is generally performed substantially at the upper limit of the ejecting frequency, a per page recording rate is reduced.
It is an object of the present invention to provide a method for effectively performing shading correction and nonejecting nozzle complementing without reducing a per page recording rate.
The present invention has been made in order to achieve the above-mentioned object, in which when corrected data during shading correction and nonejection complementing exceeds a predetermined value, complementing is performed with a different color corresponding to data-amount exceeding the maximum value.
Specifically, in both the shading correction and nonejecting nozzle complementing methods, correction processing (same color correcting) is performed using a target head as a preliminary step, and correction processing (different-color correcting) is performed using a head with a different color other than the color of the target head as a subsequent step.
Also, the predetermined value is the maximum value capable of being recorded as data.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.